Life on this planet can be said to depend on the carbon cycle in general and the processes of respiration and photosynthesis in particular. Animal respiration converts oxygen to carbon dioxide in the process of extracting energy from carbon compounds created by green plants. Green plants in their turn convert carbon dioxide into oxygen needed by animals, and in the process make the carbon compounds (mainly cellulose) required to enable both the plants and the animals to grow.
It is no accident that a balance has evolved between the plant and animal kingdoms over a few billion years of evolution. In the last 200 years however, industrialisation and the accompanying rapid growth of population, fossil fuel burning and of deforestation, have begun to affect this balance. Our survival as a species could be at stake, and as this becomes increasingly clear to the public at large, the pressures to modify "Industry" to increase our chances of survival will be enormous.
A key issue is overpopulation. The human race has proved so successful that the industrial system needed to sustain its growth is upsetting the natural cycle from which Homo sapiens evolved. If we reverse industrialisation - we would need to turn the clock back by about 200 years - the resulting human misery in terms of the lack of the basic necessities of life would be incalculable. The shortages of food, water, warmth, shelter and clothing would lead to global living standards falling to levels which would be intolerable in any country. This would certainly result in reduced population, but is clearly an unacceptable way forward, and unlikely ever to be allowed to occur. We are equally unlikely to be able to persuade families to have only one child. So, it looks as if we are stuck with a large and growing population, and must therefore turn to science to help us move towards a balanced environment maintained by a sustainable industrial system.
The fibre industry, defined in cradle-grave terms to include such sectors as polymer making, textiles and nonwovens, and their disposal methods, provide some of the basic necessities of 20th century life. Our responsibility is to try and minimise the environmental impact of the processes by which such products are manufactured.
To do this we need to make comparisons, using the best available science, of the environmental impacts of the routes to fibrous products. We must also remember that all products made form our fibres will ultimately require disposal, and that it is our duty to ensure that the disposal methods available are environmentally sound.
Such cradle-to-grave comparisons are of course complex, difficult to carry out, and beyond the scope of this paper. Here we indicate some of the environmental factors which come into play when investing in a new fibre which could become one of the major raw materials for the textiles industry in the coming decades.
THE TENCEL PROCESS
Our new fibre making process has one major raw material, the cellulose polymer, and one minor raw material, the amine oxide solvent. It also requires energy. If markets evolve along traditional lines, then some versions of the fibre will contain titanium dioxide dulling agents, and some will be bleached. All are likely to be finished with the surfactants needed by the subsequent conversion processes. Perhaps we should stress that the non- cellulosic components and bleaching options arise because the market demands them, not because they are an essential part of our process.
Cellulose
Cellulose is the natural polymer which makes up the living cells of all vegetation. It is the material at the centre of the carbon cycle, and the most abundant and renewable biopolymer on the planet. Rayon fibre producers have converted it from the fine short fibres which come from trees into the fine long fibres used by textiles and nonwovens for almost a century. Rayon nevertheless remains unique among the mass produced man-made fibres because it is the only one to use the natural polymer directly.
Polyesters, nylons, polyolefins, and acrylics all come indirectly from vegetation. They come from the polymerisation of monomers obtained from fossil fuels, which in turn are formed by the incomplete biodegradation of vegetation which grew millions of years ago.
Cellulose is produced in the cell walls of vegetation when sugars are polymerised by enzymes to form both lignin and cellulose. The sugars are produced from carbon dioxide and water by the action of sunlight on the green catalyst chlorophyll in the leaves of the plants. The industrial grade of cellulose used to make our rayon comes from tree-farms, where specially chosen species are grown from sapling to maturity in 7-10 years. New trees grow from the stumps of the cut trees, and this happens on marginal land, generally unsuitable for food crops and without the intensive use of fertilisers or pesticides. The best farms yield in excess of 2.5 tonnes of pure cellulose per acre per year. For comparison, cotton growing at its most intensive yields about 0.35 tonnes/acre and needs good soil.
Cutting down trees is popularly regarded as an unfriendly activity, and it is therefore quite important to put the usage of trees as a raw material in the correct perspective. It has been estimated (1,2) that:
- 100 billion tonnes of vegetation grow and decay annually on land. This represents about 12% of the planets total production of vegetation, the majority being produced in the oceans.
- 12% of this land-based vegetation is in the form of wood (trees).
- Of this 12 billion tonnes of wood, a maximum of 3 billion tonnes is removed by man. Half of this is burnt, either as fuel or to clear land for agriculture. The other half is used by Industry. (Compare this with 6 billion tonnes of fossil reserves "mined" each year.)
- Of the 1.5 billion tonnes of wood used by industry, half becomes timber in saw mills, and half is used raw.
- Of the 0.75 billion tonnes used raw, half goes into construction (pit-props, telegraph poles etc) and half is converted into pulp and chipboard.
- Of this 0.375 billion tonnes, 0.29 billion tonnes of wood become wood-pulp for the paper, board, fibre, film and chemicals industry.
- A significant proportion of this 290,000,000 tonnes of pulpmill feedstock (up to 40% in some areas) comes from forest thinnings, and saw mill waste and 6% from non-pulp sources such as straw, bagasse, hemp and cotton. This feedstock yields 161,000,000 tonnes of pulp.
- About 4.5 million tonnes of this pulp output are a high quality dissolving grade for forming into fibres, films, water soluble polymers and chemicals. Dissolving grade pulp is perhaps better described as industrial grade cellulose polymer, and should be considered alongside the polyester or nylon polymer beads which are the feedstocks of the synthetic fibre plants.
- Viscose rayon manufacture consumes 2,600,000 tonnes of this cellulose, with 2 million tonnes going into the staple fibre process.
Our Mobile rayon plant currently uses about 100,000 tonnes/year of the industrial grade cellulose to make viscose rayon fibres. When the first Tencel plant is on stream, an additional 18,000 tonnes/year will be required.
Cellulose Extraction (Pulping)
From the above figures, it can be seen that the 2.6 million tonnes of dissolving grade pulp currently manufactured to feed the rayon fibre industry represents 0.01% of the annual production of cellulose, on land, in nature, and about 0.7% of the cellulose in wood used by industry.
The Tencel process as currently designed will use the same sources of pulp (at slightly higher levels of efficiency) as the viscose process, but concerns related to pulp mill effluents are still with us. However I think we can by now conclude that concerns over dioxins in the pulp itself, and products made from the pulp are now behind us. Our raw material and final products, both viscose and solvent-process, have been shown by independent analysis to be free of such compounds at a detection level of 0.5 parts per trillion.
With regard to the pulp mill effluent issue, our major suppliers are undertaking programmes of work to eliminate elemental chlorine at the pulp bleaching stage, and these changes should be complete before the new fibre becomes commercial in the second half of 1992. Alternative pulping sequences which eliminate all chlorine compounds from the process are being investigated for use in both viscose and solvent systems.
Amine Oxide Solvent
N-methylmorpholine N-oxide (NMMO) is the solvent used. It is manufactured by methylation and oxidation of morpholine, which comes from a reaction between diethylene glycol and ammonia.
Whilst this is the only major chemical used in the Tencel process, its consumption is reduced to the absolute minimum by the recycling which is made possible by solvent recovery. In our Grimsby plant, which has been operating semi-commercially for the last 3 years, we have developed techniques which now recycle virtually all of the solvent used to dissolve the pulp.
Strong NMMO solution as delivered to our process has been subjected to a series of acute mammalian toxicological evaluations with conclusions as follows:
Oral LD50:
"This material is considered to be practically non-toxic by the oral route and would not be considered harmful by EEC labelling criteria."
Dermal LD50:
"This material is considered practically non-toxic by the dermal route, and would not be considered harmful by EEC labelling criteria."
Dermal Irritation:
"This material is considered to be minimally irritating to the skin, and would not be considered a skin irritant by EEC criteria."
Ocular Irritation:
"This material is considered to be minimally irritating to the eye, and would not be considered an eye irritant by EEC criteria."
In-vitro genotoxicity studies were also carried out on dilute solutions of the NMMO solvent. The test samples were inactive in Cell transformation, Mouse lymphoma forward mutation, Primary rat hepatocyte/DNA repair, and Optimised Ames assays.
In short, our solvent is harmless over the range of concentrations used in the plant, and especially so in the minimal concentrations likely to occur in any effluent.
Energy
The usage of energy and the means by which it is obtained contributes a major component of the environmental impact of most complex industrial process sequences.
The methodology of assessing the energy usage of products and processes is currently the subject of much debate, and a standardised approach has yet to emerge. Not surprisingly, most of the published work on fibres was carried out during the last energy crises in the 1973-81 period, and we could find nothing in the public domain from more recent studies.
We are aware of the following attempts (refs 3-7) to assess the total energy required to make baled staple fibre from naturally occuring raw materials, wood in the case of cellulosics and oil in the case of synthetics. In general they break the fibre production sequence into monomer making, polymer making and fibre production, and while a variety of fibres are covered, only viscose rayon and polyester are mentioned in all of them. Tonnes of fuel oil equivalent per tonne of fibre were the most popular units (TFOE/T), and Table 1 gives the values.
Table 1
Source
|
Polyester
|
Rayon
|
Woodhead 3
|
3.9
|
2.7
|
Lane and McCombes 4
|
2.6
|
2.4
|
Kogler 5
|
3.8
|
1.3
|
Armstrong 6
|
210
|
100 (base 100)
|
Marini and Six 7
|
300
|
100 (base 100)
|
CIRFS 8
|
4.2
|
1.7
|
In the same papers, nylon and acrylic fibres, where shown, require more energy than polyester (about 5 times the rayon value), and polypropylene requires less, (About 1.7 times the rayon value). Cotton requires less energy than viscose up to the bales of raw fibre, but data for the bleached and cleaned version generally needed in nonwovens is not presented.
The overall picture that emerges from these early studies was that whilst the wet-spun cellulosic fibres required more energy than melt spun polyester for the fibre making step, they had no monomer energy requirement, and the "polymerisation" requirement was minimal. In the case of the very low values for rayon emerging from Lenzing and CIRFS, we think full credit was being given for the fact that the pulp mills energy needs were in fact renewable and not dependent on fossil fuels, and that pulp could be fed directly into the viscose process without incurring any transport or drying cost. In other words, the pulp mill could be driven entirely by energy obtained from burning the parts of the tree which were not needed in the final product, and this "free" and renewable energy was not counted.
From an energy viewpoint, the solvent route to cellulosic fibres is identical to the viscose route up to the point where the cellulose enters the solvent. The energy requirements for the non-cellulosic raw materials is significantly lower in the case of the solvent route, but the solvent route will require similar energy levels in dope handling, spinning, washing and recycling. The lower water imbibition of the solvent fibre (65% versus 95%) will yield savings in fibre drying and of course in any subsequent washing and drying operations.
Overall, the solvent route will show a useful economy in this important resource when compared with viscose production on the same scale.
Fossil Reserves
Renewable resources will become increasingly important as the planets stocks of fossilised reserves are depleted and as governments realise that biomass can provide a truly sustainable, cost-effective source of energy and materials.
The viscose route currently needs fossil reserves for energy generation but for little else. At our Mobile plant the vast majority of energy requirements come from locally available natural gas.
The solvent used in the new process is made from ethylene glycol which currently comes via ethylene from oil refineries. However, as indicated above, the recycling rate is so high that solvent usage is kept down to a few kilos per tonne of fibre, and hence fossil reserve dependence is minimal.
Gaseous Effluents
The Tencel process involves direct dissolution of cellulose in a liquid which is recycled very efficiently. There are no chemical reactions and no by-products of the sort which are unavoidable in the regeneration of cellulose from the viscose route.
In the viscose process, gaseous effluent control and treatment is a fundamentally important part of the overall process and is continuously improving as the technology of the "closed-box" process evolves. The air handling and cleaning systems employed are costly and most of the emissions to atmosphere are collected and discharged through tall stacks.
The Tencel process produces very little atmospheric emission. There are traces of volatile organic compounds associated with the solvent and the soft finish which will leave the plant in the normal course of ventilation. There is no need for any central air handling or emissions stack.
Liquid Effluents
The spinning and washing liquors from the viscose process are recycled to allow reuse of the sulphuric acid and zinc sulphate components wherever this is feasible from economic and environmental standpoints. Nevertheless, in common with most industrial washing and bleaching systems, large volumes of process water have to be cleaned on-site before discharge to river. As is the case with gaseous effluents, most rayon producers stay well ahead of the regulatory requirements and this means continuously working towards improved plant designs.
The Tencel route uses much less water overall, and the process effluent needs significantly less treatment.
Disposability/Recycling
Cellulosic fibres are, as we stated at the start of this paper, simply a tiny subset of the most abundant bio-polymer on the planet.
Like natural vegetation, they can become food for micro-organisms and higher life forms (they biodegrade) and they will burn with a rather greater yield of energy than natural vegetation.
In complete biodegradation or incineration, the final breakdown products are carbon dioxide and water, and so in the overall sense these disposal methods simply recycle the cellulose to the atmospheric components from which it was made.
It is also possible to liberate and use some of the "free" solar energy which powered the polymerisation step. In the case of incineration this is straightforward in that the free-burning cellulose can be used to generate steam etc. In the case of landfill disposal, it is now well known that slow anaerobic biodegradation occurs in all landfill sites dealing with municipal solid waste. This process generates methane from cellulose, which can, and increasingly is, being used to drive gas-turbines directly. Admittedly, this process makes only a small contribution to reducing the volume of waste in the landfill, but as fuel costs rise, this "free" and renewable energy source will become more important. If landfills are designed from the start to be anaerobic bioreactors, i.e. lined and operated with moisture addition and leachate recycling, then energy generation and the return of land to normal use can be accelerated. 9
Our tests show that the anaerobic degradation process is so fast that disposables made from cellulosic fibres are likely to disappear, yielding their energy content, in the sludge digestion process used in sewage farms. I think we could all agree, that, given good mains sewage systems, and disposable designed to avoid toilet blockage, all soiled sanitary products would be better disposed of by flushing. It is only the current need for some non-cellulosic components which prevents this ideal being attained.
CONCLUSION
This paper discussed some of the environmental issues which have to be considered when developing new production plant for textile and nonwovens raw materials.
Rayon fibres, made for a century by the direct conversion of abundant vegetable matter, have always had much to recommend them in textiles compared with synthetics made from fossil fuels. The renewability of their main raw material, their overall energy efficiency, their lack of dependence on fossil fuels, their long history of safe use in hygiene applications, and their easy disposal and natural recyclability make them strong contenders for tomorrow's textile industry also.
The new Tencel route to rayon reinforces these inherent strengths by using a modern fibre production system which, being physical rather than chemical, reduces environmental impacts to a minimum. The Tencel investment, coupled with the continuous improvement of the traditional route, gives us what we believe is a winning approach to textile industry fibre supply for some time to come. The new plant is due to be on-stream at Mobile Alabama in mid 1992.
References:
1 "Eco-profiling of cellulose-based products", August 1990. (A study prepared for Courtaulds and others by Envirocell - 256 references).
2 John Emsley (Kings College London) - "Plant a tree for Chemistry", published in New Scientist, 8th October 1987.
3 Woodhead; ICI; International TNO Conference; 1976.
4 Lane and McCombes; Courtaulds; Textile Manufacturer No 1; 1979.
5 Kogler; Lenzing; EDANA AGM; Munich 1980.
6 Armstrong; Consultant; EDANA AGM; Munich 1980.
7 Marini and Six; EDANA Nonwovens Symposium Milan, 1985.
8 Unpublished Data; Interantional Committee for Rayon and Synthetic Fibres; 1982
9 Pohland and Cross, "Controlled Landfill Management - Principles and Applications"; Insight 91, Charleston, Oct 91.
Calvin Woodings
Research Fellow
Courtaulds Research, Coventry
UK
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